Field sequential color display apparatus

ABSTRACT

A field sequential color display apparatus for sequentially displaying RGB field images so that a color frame image can be observed, comprising: an LCD display device for displaying RGB field images; a timing generation/control circuit for controlling the order of RGB field images to be displayed on the LCD display device; a pixel shifting unit for causing the beam of each pixel displayed on the LCD display device to be shifted by each frame image so as to enhance the resolution of image observed by a viewer; and an optical system for making it possible to display an image before shifted at the pixel shifting unit and an image after the shift. Among the RGB field images, the G field image having highest spectral luminous efficiency is displayed at an intermediate order within the frame images so that leakage thereof is not caused by the pixel shifting operation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. Ser. No. 10/297,288 filed Dec. 4,2002 now U.S. Pat. No. 7,002,539, which is a U.S. National PhaseApplication under 35 USC 371 of International Application PCT/JP02/02009filed Mar. 5, 2002.

TECHNICAL FIELD

The present invention relates to field sequential color displayapparatus in which high-resolution color images are displayed by using apixel shifting unit for shifting pixels by means of optical wobblingoperation.

BACKGROUND OF THE INVENTION

Among image display apparatus using a liquid crystal display device orthe like, an image display apparatus has been disclosed for example inJapanese patent applications laid open No. 6-324320 and No. 7-7704 inwhich resolution of the liquid crystal display device is improved byeffecting a pixel shifting operation called wobbling where the opticalaxis of light from the liquid crystal display device is wobbled inpredetermined directions.

A description will now be given with respect to the general constructionof an image display apparatus in which resolution is improved by suchoptical wobbling operation. As shown in FIG. 1, a back light 102 foremitting white light is placed on the back side of a color liquidcrystal display device 101, and a wobbling device (a pixel shiftingunit) 103 for wobbling in predetermined directions the optical axis oflight from the color liquid crystal device 101 is placed on the frontside of the color liquid crystal display device 101. Here, odd fieldimages and even field images of input video signal are displayed on thecolor liquid crystal display device 101 at the same pixels thereofthrough an image display control circuit 104. In accordance with theirdisplay timing, the optical axis of light from the color liquid crystaldisplay device 101 is wobbled in predetermined directions by thewobbling device 103.

The wobbling device 103 includes a polarization changing liquid crystalplate 105 and a birefringence plate 106 which is placed on the frontside thereof. Here, ON/OFF of voltage across the polarization changingliquid crystal plate 105 is controlled by a wobbling liquid crystaldrive circuit 107 based on synchronizing signal of the video signal tobe displayed on the color liquid crystal display device 101. The lightfrom the color liquid crystal display device 101 is thereby transmittedwithout changing its polarization when the voltage is ON, while, whenthe voltage is OFF, the light from the color liquid crystal displaydevice 101 is transmitted with changing its polarization through 90degrees, effecting the wobbling operation by changing the location to beemitted from the birefringence plate 106 in accordance with suchdirection of polarization. It should be noted that, since the colorliquid crystal display device 101 retains the image of the precedingfield until rewriting of the image of the next field, one of theelectrodes of the polarization changing liquid crystal plate 105 isdivided into parts each with a plurality of lines such as 5 lines. Theother electrode is used as a common electrode and application of voltageis controlled by selecting the one of the electrodes in accordance withthe timing of line scan of the color liquid crystal display device 101.

The following operation is performed when alternately displaying oddfield images and even field images on the color liquid crystal displaydevice 101. In particular, a case is supposed here as shown in FIG. 2Athat the horizontal pixel pitch is Px and the vertical pixel pitch is Pyof a pixel group in delta array of the color liquid crystal displaydevice 101. An oblique wobbling operation of 0.75 Px in the horizontaldirection and 0.5 Py in the vertical direction, for example, isperformed by the above described wobbling device 103 so that the pixelarray of the color liquid crystal display device 101 is located at theposition as indicated by the broken lines in FIG. 2B when an odd fieldimage is to be displayed, while the pixel array is located at theposition indicated by solid lines when an even field is to be displayed.Specifically, if for example Px is 18 μm and Py is 47.5 μm, the wobblingoperation is effected so as to achieve an oblique distance of about 27.3μm, shifted by 13.5 μm horizontally and 23.75 μm vertically.

For this reason, a crystallographic axis 106 a of the birefringenceplate 106 is set as shown in FIG. 3 in a direction inclined with respectto the XY coordinate of on the color liquid crystal display devicesurface and Z direction which is normal thereto. Here, when thedirection of polarization of incidence agrees with the direction ofpolarization of light from the color liquid crystal display device, thelight from the color liquid crystal display device is transmitted asextraordinary rays so as to shift the pixels. When the direction ofpolarization of incidence is rotated through 90 degrees with respect tothe direction of polarization of light from the color liquid crystaldisplay device, it is transmitted intact as ordinary rays withoutshifting the pixels.

In this manner, as shown in FIG. 4, when the image of an odd field is tobe displayed on the color liquid crystal display device 101, voltageapplication to the region of the polarization changing liquid crystalplate 105 corresponding to the horizontal lines to be rewritten isturned ON, so as to transmit the light from such lines intact withoutrotating the direction of polarization through 90 degrees. The light isemitted by the birefringence plate 106 as extraordinary rays to shiftthe pixels. On the other hand, when the image of an even field is to bedisplayed, voltage application to the region of the polarizationchanging liquid crystal plate 105 corresponding to the horizontal linesto be rewritten is turned OFF, so as to transmit the light from thelines as rotated in the direction of polarization through 90 degrees,causing the birefringence plate 106 to emit the light intact as ordinaryrays without shifting the pixels.

In addition, an image display apparatus is known to be provided with twounits of such one-dimensional two-point pixel shifting unit each havinga polarization changing liquid crystal plate and birefringence platewhich are combined into a laminate where one of the units is rotatedthrough 90 degrees about the axis of incident light with respect to theother so that a high resolution by two-dimensional four-point pixelshifting is achieved by performing four times of pixel shift in thevertical and horizontal directions within one frame or one field.

Further, though not related to high-resolution display using a pixelshifting unit, a field sequential color display apparatus having aconstruction as will be described below is disclosed in Japanese patentapplication laid open No. 8-248382. Specifically, the field sequentialcolor display apparatus uses a monochrome CRT and a liquid crystalshutter (color filter) consisting of π-cell and color polarizing plate.To reduce color mixture that occurs between fields due to delay in theresponse speed of π-cell contained in the liquid crystal shutter, theswitching timing of ON/OFF of the liquid crystal shutter is set within ablanking period occurring between two image periods of sequential colorsignals and the switching timing of ON/OFF is set so as not to causecolor mixture.

If an attempt is made to display a high-resolution color image byapplying the above described pixel shifting unit to a field sequentialcolor display apparatus for effecting color displaying by switchingamong trichromatic images according to time, the following problemoccurs. In particular, though there is not much problem when the liquidcrystal cell is turned ON from OFF, the problem occurs when it is turnedOFF from ON that pixels are seen to be displayed simultaneously at twolocations in such transition period, i.e., at the original pixellocation and at the pixel location shifted by pixel shifting. A colorleakage is thereby caused.

In the above laid-open publication, a disclosure is made merely withrespect to color mixture in the field sequential color display apparatususing a liquid crystal shutter consisting of π-cell and color polarizingplate as the color filter. It fails to mention the occurrence of colorleakage in a high-resolution field sequential color display apparatususing a pixel shifting unit.

To eliminate the above problems in achieving a display at highresolution by using a pixel shifting unit in field sequential colordisplay apparatus, it is an object of the invention to provide a fieldsequential color display apparatus in which sensory color mixture due tocolor leakage arising from the shifting of pixel can be reduced.

DISCLOSURE OF THE INVENTION

In accordance with a first aspect of the invention, there is provided afield sequential color display apparatus for sequentially displayingimage information as field images by each of a plurality of colorinformation so that a viewer can observe a color frame image, including:display means for displaying the field images; display control means forcontrolling the order of the field images to be displayed on the displaymeans; pixel shifting means containing a liquid crystal cell for causingthe beam of each pixel displayed on the display means to be shifted byeach frame image so as to enhance the resolution of image observed bythe viewer; and optical means for making it possible to display to theviewer an image before shifted at the pixel shifting means and an imageafter the shift. The display control means effects control so that thefield image of color information having highest spectral luminousefficiency in the color information is displayed on the display means atan intermediate order except the first and last within the frame images.

In order to reduce sensory color mixture of the viewer in the case whereshifting by the pixel shifting means is effected by each frame image toachieve a high resolution in field sequential color display apparatus,it is necessary to eliminate leakage (appearance of one pixel as two) ofthe color information image of the highest spectral luminous efficiencyarising from the shifting of pixel. When shifting by the pixel shiftingmeans is effected by each frame image, color leakage occurs in the firstand last field images within frame images. No color leakage occurs in amedium field image. Accordingly, as described above, it is possible toeliminate leakage arising from pixel shifting so as to reduce sensorycolor mixture of the viewer such that the color information image ofhighest spectral luminous efficiency such as G field image having thehighest spectral luminous efficiency for example in the case where thethree colors of RGB are used as the plurality of color information, isdisplayed at an intermediate order except the first and last.

In accordance with a second aspect of the invention, there is provided afield sequential color display apparatus for sequentially displayingimage information as field images by each of a plurality of colorinformation so that a viewer can observe a color frame image, including:display means for displaying the field images; display control means forcontrolling the order and displaying time of the field images to bedisplayed on the display means; pixel shifting means containing a liquidcrystal cell for causing the beam of each pixel displayed on the displaymeans to be shifted by each frame image so as to enhance the resolutionof image observed by the viewer; and optical means for making itpossible to display to the viewer an image before shifted at the pixelshifting means and an image after the shift. The display control meanseffects control so that, of the plurality of color information displayedduring a transition period of shifting of the ray by the operation ofthe pixel shifting means, the displaying time of field image of thecolor information having highest spectral luminous efficiency is shorterthan the displaying time of field images of the other color information.

In this manner, leakage of the color of higher spectral luminousefficiency of the colors to be displayed during the transition time ofpixel shifting can be reduced so as to reduce sensory color mixture ofthe viewer by effecting control such that, among the plurality of colorinformation to be displayed during the transition period for shiftingray of each pixel by the operation of the pixel shifting means, thedisplaying time of a field image of the color information of highestspectral luminous efficiency is shorter than the displaying time of afield images of other color information.

In accordance with a third aspect of the invention, the field sequentialcolor display apparatus according to the second aspect further includestemperature measurement means for measuring an ambient temperature ofthe pixel shifting means containing the liquid crystal cell, wherein thedisplay control means controls the displaying time of field image ofeach color information and the driving timing at which the pixelshifting means shift the ray, on the basis of temperature measured bythe temperature measurement means.

By thus providing the temperature measurement means to control thedisplaying time of field image of each color information and the drivingtiming of the pixel shifting means on the basis of measuredtemperatures, it becomes possible to control the displaying time offield image of each color information and the driving timing of thepixel shifting means correspondingly to change in the transition periodof the pixel shifting means arising from change in temperature. Areduction of sensory color mixture can be achieved more suitablycorresponding to change in temperature.

In accordance with a fourth aspect of the invention, there is provided afield sequential color display apparatus for sequentially displayingimage information as field images by each of a plurality of colorinformation so that a viewer can observe a color frame image, including:display means for displaying the field images; display control means forcontrolling the order of the field images to be displayed on the displaymeans; pixel shifting means containing a liquid crystal cell for causingthe beam of each pixel displayed on the display means to be shifted byeach frame image so as to enhance the resolution of image observed bythe viewer; and optical means for making it possible to display to theviewer an image before shifted at the pixel shifting means and an imageafter the shift. The display control means causes the color informationof the field image displayed before and that after switching of frameimage to be identical with each other.

By thus causing the field images displayed before and after theswitching of frame image to have an identical color information, it ispossible to avoid an occurrence of color mixture arising from pixelshifting at the time of switching of frame image.

In accordance with a fifth aspect of the invention, the field sequentialcolor image display apparatus according to the fourth aspect ischaracterized in that the color information of the field images causedto be identical is sequentially changed.

In the case where the same color information becomes visible at eachswitching of frame image, a flicker is caused due to the fact that theinterval of such same color information becoming visible is wider thanthe interval of other color information. Such flicker can be reduced,however, by sequentially changing at each switching of frame image thecolor information of which color mixture at the time of the switching offrame image is prevented as described above.

In accordance with a sixth aspect of the invention, there is provided afield sequential color display apparatus for sequentially displayingimage information as field images by each of a plurality of colorinformation so that a viewer can observe a color frame image, including:display means for displaying the field images; display control means forcontrolling the order of the field images to be displayed on the displaymeans; pixel shifting means containing a liquid crystal cell for causingthe beam of each pixel displayed on the display means to be shifted byeach frame image so as to enhance the resolution of image observed bythe viewer; and optical means for making it possible to display to theviewer an image before shifted at the pixel shifting means and an imageafter the shift. The display control means causes one field perioddisplayed before or after switching the frame image to be anon-displaying field period during which nothing is displayed.

By such construction, no image is displayed on the display means in theone field period at ON-to-OFF or OFF-to-ON operation of the liquidcrystal cell of the pixel shifting means. It is thereby possible toavoid an occurrence of color mixture due to leakage arising from theshifting of pixel.

In accordance with a seventh aspect of the invention, there is provideda field sequential color display apparatus for sequentially displayingimage information as field images by each of three colors of colorinformation so that a viewer can observe a color frame image, including:display means for displaying the field images; display control means forcontrolling the order of the field images to be displayed on the displaymeans; pixel shifting means containing a liquid crystal cell for causingthe beam of each pixel displayed on the display means to be shifted byeach frame image so as to enhance the resolution of image observed bythe viewer; and optical means for making it possible to display to theviewer an image before shifted at the pixel shifting means and an imageafter the shift. The display control means selects field images of twocolors from the field images of the three colors to form one frame imageand effects control so as to cause the field images of the three colorsto be displayed at least once in every two frame images.

In this manner, image information of two colors is selected from theimage information of the three colors for forming a color image to formone frame image and the field images of the three colors are displayedat least once in every two frame images so as to make it possible toreduce the number of fields to be displayed in one frame image. It isthereby possible to make slower the operation speed of the display meansfor displaying field images and it becomes easier to display the fieldimage having high spectral luminous efficiency alone at high resolution.

In accordance with an eighth aspect of the invention, the displaycontrol means of the field sequential color display apparatus accordingto the seventh aspect always selects a green field image as the fieldimage of one color in the field images of the two colors which areselected to form one frame image and further includes a white balanceadjusting means using signal values of two green field images that areclose to each other in the displaying order so as to calculate a signalvalue of green field image to be displayed after these so that aninconsistency in white balance due to the fact that green field imagesare displayed at a greater number of times does not occur.

When, as in the seventh aspect, one frame image is formed by selectingimage information of two colors from the image information of the threecolors for forming a color image and at the same time the field imagesof the three colors each are displayed at least once in every two frameimages, it results in the color information of one color alone of theimage information of the three colors being selected for every one frameso that inconsistency is caused in white balance. To eliminate this, inthe eighth aspect, the white balance adjusting means is provided as theabove to adjust the signal value of green field image that is selectedfor every one frame. It is thereby possible to adjust inconsistency inwhite balance.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 schematically shows an image display apparatus using aconventional wobbling device (pixel shifting unit).

FIGS. 2A and 2B show the manner of pixel array by the wobbling operationof a color liquid crystal display device.

FIG. 3 illustrates the operation of a birefringence plate of thewobbling device.

FIG. 4 shows the manner of pixel shifting in an odd field and even fieldby the wobbling device of the image display apparatus shown in FIG. 1.

FIGS. 5A to 5C schematically show the optical construction and themanner of pixel shifting in an embodiment of the field sequential colordisplay apparatus according to the invention.

FIG. 6 is a block diagram showing the construction of electricalcircuits for generating for example driving signals to be supplied toeach section of the field sequential color display apparatus shown inFIG. 5A.

FIG. 7 shows timings for performing a standard field sequential colordisplaying in the field sequential color display apparatus shown inFIGS. 5A and 6.

FIG. 8 shows response characteristics of the liquid crystal cell of thepixel shifting unit.

FIG. 9 explains the manner of occurrence of color leakage and colormixture arising from the pixel shifting operation of the pixel shiftingunit.

FIGS. 10A and 10B explain manners of reducing leakage and color mixtureby adjustment of the lighting timing of LED light source and the drivingtiming of the liquid crystal cell.

FIG. 11 explains another manner of effecting reduction in leakage andcolor mixture by adjustment of the lighting timing of LED light sourceand the driving timing of the liquid crystal cell.

FIG. 12 shows the manner of the lighting timing of LED light source andthe driving timing of the liquid crystal cell in the case where thetransition period of liquid crystal cell becomes shorter due to a risein temperature.

FIG. 13 explains the manner of effecting reduction in leakage and colormixture due to pixel shifting operation by adding a non-displaying fieldto form 1-frame 4-field structure.

FIG. 14 shows the manner of making shorter the displaying period of thenon-displaying field in the 1-frame 4-field structure.

FIG. 15 explains the technique of reducing color mixture arising fromthe pixel shifting operation by changing the displaying order of RGBfield images of the field sequential color displaying.

FIG. 16 is a timing chart for explaining a high-resolution display inthe field sequential color displaying by 2-frame 4-field structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be given by way of the attached drawings toexplain the invention in detail. FIG. 5A schematically shows the opticalconstruction of an embodiment of the field sequential color displayapparatus according to the invention. Included in FIG. 5A are: 1, alight source consisting of LED for emitting illuminating light of thecolors of R, G, B; 2, a polarizing beam splitter (PBS); 3, a reflectingtype LCD display device; 4, a pixel shifting unit, i.e., 2-point pixelshifting unit consisting of a liquid crystal cell 4-1 and abirefringence plate 4-2; and 5, an optical system.

In thus constructed field sequential color display apparatus, R, G, Bfield images are sequentially displayed on the reflecting type LCDdisplay device 3 and at the same time an illuminating light from theRGB-LED light source 1 is caused to irradiate the reflecting type LCDdisplay device 3 through the beam splitter 2 so that R, G, B fieldimages enter the pixel shifting unit 4 through the beam splitter 2. Bymeans of ON/OFF control at each frame image of the liquid crystal cell4-1 of the pixel shifting unit 4, these are then displayed through theoptical system 5 at the pixel array location as indicated by dottedlines in FIG. 5C which is pixel-shifted from the displaying of theoriginal LCD display device pixel array shown in FIG. 5B. Ahigh-resolution color displaying is thereby performed.

FIG. 6 is a block diagram showing an electrical circuit construction togenerate such as driving signals to be supplied to each section of thefield sequential color display apparatus having the construction asdescribed. FIG. 6 includes: 11, A/D conversion circuit for effecting A/Dconversion of input image signals; 12, a field sequential displayconversion circuit for tripling the rate of the A/D converted imagesignal to separate it into R, G, B signals so as to output R, G, B fieldsequential signals; 13, D/A conversion circuit for effecting D/Aconversion of the R, G, B field sequential signals from the fieldsequential display conversion circuit 12; and 14, LCD driving circuit towhich the D/A-converted R, G, B field sequential signals are inputted togenerate driving signal for driving the reflecting type LCD displaydevice 3.

It further includes: 15, a timing generation/control circuit to which asynchronizing signal of the input image signal is inputted to generatetiming signals to the respective sections and to control each section;16, PLL circuit for controlling the timing generation/control circuit15; 17, LED driving circuit to which a timing signal from the timinggeneration/control circuit 15 is inputted to generate a driving signalfor driving the RGB-LED light source 1; 18, a liquid crystal celldriving circuit to which a timing signal from the timinggeneration/control circuit 15 is inputted to generate a driving signalfor controlling ON/OFF of the liquid crystal cell 4-1 of the pixelshifting unit 4; and 19, a temperature sensor for detecting ambienttemperature of the liquid crystal cell 4-1, based on the detected valueat which the timing generation/control circuit 15 controls the drivingtiming of RGB-LED light source 1 and the liquid crystal cell 4-1. Inaddition, the timing generation/control circuit 15 controls such as theorder of the color field images and the number of field images in eachfield image.

A description will now be given to the operation method which featuresthe present invention in the field sequential color display apparatushaving the construction as described. First, the field sequential colordisplay operation will be described by way of the timing chart in FIG.7. At the same time of displaying field images by sequentially applyingR, G, B field sequential field signals to the reflecting type LCDdisplay device 3 through the LCD driving circuit 14, the LED lightingsignals for lighting LED light source 1 of each of RGB colors areprovided from the LED driving circuit 17 correspondingly to the fieldimages. Field sequential color images are thereby obtained on thereflecting type LCD display device 3.

When pixel shifting is performed for each frame image by the pixelshifting unit 4 with respect to the field sequential color imagesobtained in the above manner from the reflecting type LCD display device3, a liquid crystal cell driving signal of which ON/OFF is controlled asshown in FIG. 8 is provided to the liquid crystal cell 4-1 from theliquid crystal cell driving circuit 18. Based on this driving signal,especially because of response delay at the time of ON to OFF, theliquid crystal cell 4-1 requires a response delay time t₁ and transitionperiod t₂ until the liquid crystal cell in ON state is completely turnedinto OFF state. Accordingly, during the transition period until theoriginal pixel location is displayed in a completely shifted manner bythe pixel shifting operation, pixels are displayed at two locations,causing a displaying leakage. If different colors are displayed at thetwo locations, a color mixture is caused.

The state of occurrence of such color mixture will now be described indetail. As shown in FIG. 9, when B-LED light source for example is litin a beginning period of the transition period of the response operationof the liquid crystal cell, blue display is leaked to OFF-location atthe rate of the slanting line portion. Further, when R-LED light sourceis lit in an ending period of the transition period, red display isleaked to ON-location at the rate of the slanting line portion.

When shifting by pixel shifting unit is effected at each frame image inthe field sequential color display apparatus, a color leakage occurs inthe first and last field images within frame image, i.e., in the B fieldsignal and R field signal in the example shown in FIG. 7. No colorleakage occurs in G field image at the middle thereof. To reduce sensorycolor mixture of the viewer, it is necessary to eliminate a leakageresulting from the pixel shifting operation of the color informationimage having highest spectral luminous efficiency. When the threeprimary colors of RGB are used as the color information as in theexample shown in FIG. 7, the color of G has the highest spectralluminous efficiency. For this reason, G field image is displayed at themiddle except the first and last as shown in FIG. 7, i.e., as the secondfield of each frame. A leakage by the pixel shifting operation of Gfield image having the highest spectral luminous efficiency can thus beeliminated to reduce sensory color mixture of the viewer.

In the description of the above first operation method, color mixture isreduced by setting the G field signal having the highest spectralluminous efficiency as the second frame signal so as to eliminateleakage resulting from the pixel shifting operation thereof. On theother hand, it is also possible to eliminate leakage to reduce colormixture by means of adjustment of the timing at which LED light source Iis lit and the timing of ON/OFF of the liquid crystal cell 4-1.

In particular, in a normal case as shown in FIG. 10A, the lightingsignal of B-LED light source 1 is turned ON in a beginning period of theON-to-OFF transition period in the response operation of the liquidcrystal cell 4-1 of the pixel shifting unit 4, and the lighting signalof R-LED light source 1 is ON in an ending period of the transitionperiod. With such lighting operation condition of LED light source andON/OFF timing of the liquid crystal cell, however, both B field image ofn−th frame and R field image of (n+l)−th frame penetrate the transitionperiod and are leaked into each other to be displayed in a state ofcolor mixture.

In another operation method of the invention, therefore, since the Rfield image has a higher spectral luminous efficiency than the B fieldimage, ON/OFF-timing of the liquid crystal cell and the lighting timingof R-LED light source are regulated as shown in FIG. 10B so as not tolight R-LED light source to the extent possible in the transition periodwhere leakage occurs so that leakage is not caused in the R field imagewhich has such higher spectral luminous efficiency. In particular, thelighting timing of B-LED light source is set with a priority on thelighting of R-LED light source to adjust the emitting interval of LEDlight source (LED emission cycle). In the example shown in FIG. 10B,ON/OFF-timing of the liquid crystal cell is somewhat advanced and at thesame time the lighting timing of B-LED light source is advanced to widenthe LED emitting interval. A leakage of R field image can thus bereduced or eliminated to reduce or prevent an occurrence of colormixture. In this connection, the emitting timing of RGB-LED light sourceis preferably in an ending period of the displaying period due to theresponse characteristics of the LCD display device.

Further, in the-case as shown in FIG. 11 where the displaying order ofthe field sequential color field images at the reflecting type LCDdisplay device 3 is converse to the case of FIG. 10, i.e., in the casewhere the last field of n−th frame is the R field image and the firstfield of (n+l)−th frame is the B field image, the timing is set alsowith a priority on the displaying of the R field image which has ahigher spectral luminous efficiency. In particular, the lighting periodof R-LED light source for displaying the R field image is set so that itdoes not enter the transition period of the liquid crystal cell to theextent possible. It is thereby possible to reduce leakage of the R fieldimage so as to reduce sensory color mixture.

Now, the transition period of the liquid crystal cell 4-1 of the pixelshifting unit 4 is highly dependent on temperature and the transitionperiod fluctuates. Accordingly, it is possible to more suitably reduceleakage so as to reduce color mixture by setting the timingscorrespondingly to the fluctuation in the transition period of theliquid crystal cell due to temperatures. In regulating the timings, anambient temperature of the liquid crystal cell is detected at thetemperature sensor 19 so that, based on such detected value, thelighting timing of LED light source 1 and the driving timing of theliquid crystal cell 4-1 are regulated at the timing generation/controlcircuit 15.

FIG. 12 shows the case where the transition period of the liquid crystalcell 4-1 becomes shorter due to a temperature rise. The lighting timingin this case, even if the same as the lighting timing shown in FIG. 10A(the case of lighting in an ending period of each field image signal),is capable of making shorter the period during which the lighting periodof R-LED light source enter the transition period so that leakage can bereduced. In other words, when the transition period has become shorter,it is possible to prevent an occurrence of color mixture even if thelighting timing of B-LED light source is delayed so as to display Bfield image which is more like B.

While, in the above described operation methods, the image signal hasbeen shown as of 1-frame 3-field structure, a description will now begiven with respect to an operation method for preventing leakage andreducing color mixture by 1-frame 4-field structure where anon-displaying field is provided. In this method, as shown in the timingchart of FIG. 13, 1-frame 4-field structure is used and a field of themode referred to as BL (black) is provided at the last or fourth fieldso that the fourth field in such BL mode is used as the non-displayingfield. In this case, LED light source is not lit and a dark portionresults during such non-displaying field period so that, even if leakageoccurs due to the pixel shifting operation at each frame, an effect dueto such leakage is not produced. Accordingly, the state of furtherreduced leakage can be achieved in the pixel shifting operation.

When the effective displaying periods of the respective fields are thesame with each other in the operation method where the image signal isformed as 1-frame 4-field structure as described, it is possible thatthe images of the three fields of R, G, B before the added field becomeless visible due to the effect of the response speed of the LCD displaydevice 3. Here, since it is not necessary that the added fourth field in1-frame 4-field structure has the same displaying period as each of theother fields, the displaying period of the fourth field can be somewhatreduced as shown in FIG. 14. It is thereby possible to provide a marginin the operation speed of LCD display device in the effectivelydisplayed three fields of RGB.

While the fourth field is formed as a non-displaying field in theoperation method using the above described 1-frame 4-field structure, asimilar effect and advantage can be obtained also by using the firstfield as a non-displaying field. A similar effect and advantage can alsobe achieved, without providing a non-displaying period as the fourthfield in the above described manner, by providing a blanking period(black displaying period:period for displaying nothing) toward theswitching point within the displaying fields located before and afterthe switching point of frame image (pixel shifting point) in time. Inthis case, the field period of field having a blanking period becomeslonger correspondingly to the blanking period than the field period offield without a blanking period.

In each of the above described operation methods, a description has beengiven with respect to a method of reducing an occurrence of leakage andcolor mixture in the image signal of 1-frame 3- or 4-field structurehaving a specified displaying order of the color field images in eachframe. A description will now be given with respect to method forpreventing color mixture by changing the displaying order of therespective color field images in each frame of 1-frame 3-fieldstructure.

FIG. 15 explains the method for preventing color mixture by changing thedisplaying order of the color field images in each frame of the imagesignal having 1-frame 3-field structure. In a standard operation infield sequential color display system where the image signal is of1-frame 3-field structure, if three primary colors of RGB are used, aspecified order such as RGB, RGB . . . is used as the displaying orderin each frame. In the method of the invention, on the other hand, thedisplaying order of RGB field images is changed correspondingly to thepixel shifting operation at each frame so that field images of the samecolor are consecutively displayed before and after the switching of eachframe.

In the illustrated example, the field displaying order is changed ateach frame so that fields of the same color are located before and afterthe switching of frame such as RGB, BGR, RGB, BGR . . . By thus changingthe displaying order of RGB field images in each frame, an occurrence ofcolor mixture can be avoided even when leakage is caused by the pixelshifting operation at the switching of frame, since it is of the samecolor.

If field images of the same color are displayed before and after theswitching of frame as described above, the displaying interval of thecolor to be consecutively displayed at the switching becomes wider thanthe displaying interval of other colors so as to result in a state whereflicker occurs, though an occurrence of color mixture is prevented dueto the consecutive displaying. A method for mitigating such flickerstate will now be described.

In the present invention, the pixel shifting operation is performed bymeans of ON/OFF of the liquid crystal cell at the time of switching offrame in the field sequential color display apparatus. The problem ofoccurrence of leakage and color mixture resulting from pixel shiftingoccurs at the switching timing of ON to OFF of the liquid crystal cell.Instead of consecutively displaying field images of the same colorbefore and after the switching of frame at every switching of frame,therefore, an occurrence of color mixture can be prevented whilereducing the occurrence of the above described flicker by setting thedisplaying order of field images so that field images of the same colorare displayed before and after the switching of frame only when theliquid crystal cell is switched from ON to OFF.

A description will now be given by way of the timing chart of FIG. 16with respect to an embodiment of the case of applying the pixel shiftingmethod by pixel shifting unit to field sequential color display systemof 2-frame 4-field structure where the field images of the three colorsare displayed at least once in every two frames. In the illustratedexample, of image signal of 2-frame 4-field structure, only G fieldsignal having high spectral luminous efficiency is displayed in eachframe so that such G field signal having high spectral luminousefficiency alone is displayed at high resolution by the pixel shiftingoperation of pixel shifting unit.

In the case of such construction, since the image signal is of 2-frame4-field structure, it is possible to make slower the operation speed ofLCD display device 3 so as to provide a margin. Further only thedisplaying of G having high spectral luminous efficiency can be effectedat high resolution.

In 2-frame 4-field structure, if as described only G field image isdisplayed in every frame i.e., twice in two frames so as to be displayedat a greater number of times than the displaying of other R, B fieldimages, an inconsistency occurs in white balance. To be matched with theother colors by eliminating such inconsistency in white balance, thevideo signal to be supplied to LCD display device at each G field is setfor each pixel by computation from G signals of two frames. It isthereby possible to perform a high-resolution displaying withmaintaining a white balance.

A description will now be given with respect to the setting of videosignal to be supplied to LCD display device at each G field from Gsignals of two frames. Supposing the input level of odd-frame G signalas Godd and the input level of even-frame G signal as Geven and alsosupposing the output levels of G video signals to be supplied to LCDdisplay device in odd and even frames as L(Godd) and L(Geven),respectively, these are expressed as follows.If Godd≧Geven:ΔG=Godd−GevenL(Godd)=k1{(Godd−ΔG)/2}+ΔGL(Geven)=k2(Geven/2)If Godd<GevenΔG=Geven−GoddL(Godd)=k1(Godd/2)L(Geven)=k2{(Geven−ΔG)/2}+ΔGwhere k1, k2 are factors to be set in accordance with thecharacteristics of light source and pixel shifting unit. Such as thesettings of respective signal levels for the above described whitebalance adjustment are all performed by the timing generation/controlcircuit and by the correction of RGB image signal levels.

While an example using a reflecting type as the LCD display device hasbeen shown in the above described embodiments, it is naturally alsopossible to use a transmitting type LCD display device which uses a backlight.

EFFECT OF THE INVENTION

As has been described, since the field sequential color displayapparatus according to the first aspect of the invention is constructedto display the field image of color information having highest spectralluminous efficiency at an intermediate order in frame image, it becomespossible to reduce an occurrence of sensory color mixture by eliminatingleakage arising from the shifting of pixel of the color informationimage having highest spectral luminous efficiency. In the image displayapparatus according to the second aspect of the invention, since, of theplurality of color information to be displayed during the transitionperiod for shifting the beam by the pixel shifting means, the displayingtime of field image signal of the color information having the highestspectral luminous efficiency is shorter than the displaying time of theother color information, the leakage of color of higher spectralluminous efficiency can be reduced so as to reduce sensory colormixture. In the field sequential color display apparatus according tothe third aspect of the invention, since the displaying timing of eachcolor field image and the driving timing of the pixel shifting means arecontrolled on the basis of temperatures measured by the temperaturemeasurement means, it becomes possible to control the displaying timingof each color field image and the driving timing of the pixel shiftingmeans correspondingly to changes in the transition period of the pixelshifting means due to temperature change so as to more suitably reducesensory color mixture corresponding to temperature change.

In the field sequential color display apparatus according to the fourthaspect of the invention, since the field images displayed before andafter switching of frame image are made to have the same colorinformation with each other, it is possible to avoid an occurrence ofcolor mixture arising from pixel shifting occurring when the frame imageis switched. In the field sequential color display apparatus accordingto the fifth aspect of the invention, since the color information of thefield images to be identified before and after switching of frame imageis sequentially changed, the identified color information can bedispersed to reduce flicker. In the field sequential color displayapparatus according to the sixth aspect of the invention, since onefield period displayed before or after switching of frame image iscaused to be a non-displaying field period during which nothing isdisplayed, it is possible to prevent an occurrence of color mixture dueto leakage arising from the shifting of pixel. In the field sequentialcolor display apparatus according to the seventh aspect of theinvention, since two colors are selected from the image information ofthree colors for constituting a color image to form one frame image andthe field images of the three colors are displayed at least once inevery two frames, it is possible to reduce the number of fields to bedisplayed in one frame so that the operation speed of the display meanscan be made slower and it becomes possible to display the color fieldimage having high spectral luminous efficiency alone at high resolution.In the field sequential color display apparatus according to the eighthaspect of the invention, G field image having high spectral luminousefficiency can be displayed at high resolution while adjustinginconsistency in white balance.

1. A field sequential color display apparatus for sequentiallydisplaying field images corresponding to a plurality of colors todisplay a plurality of color frame images, wherein each of the fieldimages is a single-color image and each of said color frame images isformed from a plurality of the field images, said field sequential colordisplay apparatus comprising: display means for sequentially displayingthe single-color field images one-by-one; display control means forcontrolling a display order in which the field images are displayed bythe display means; pixel shifting means, including a liquid crystalcell, for shifting alternating ones of the color frame images, byshifting a beam from each pixel displayed on the display means for ashifted one of the color frame images; and optical means fortransmitting to a viewer both shifted color frame images and unshiftedcolor frame images; wherein the display control means controls displayof the field images so as to minimize, to a greatest possible extent, anoverlap amount of a transition period of the pixel shifting means duringwhich the beam of said each pixel is shifted with display periods ofeach of the field images.
 2. A field sequential color display apparatusfor sequentially displaying field images corresponding to a plurality ofcolors to display a plurality of color frame images, wherein each of thefield images is a single-color image and each of said color frame imagesis formed from a plurality of the field images, said field sequentialcolor display apparatus comprising: display means for sequentiallydisplaying the single-color field images one-by-one; display controlmeans for controlling a display order in which the field images aredisplayed by the display means; pixel shifting means, including a liquidcrystal cell, for shifting alternating ones of the color frame images,by shifting a beam from each pixel displayed on the display means for ashifted one of the color frame images; and optical means fortransmitting to a viewer both shifted color frame images and unshiftedcolor frame images; wherein display periods of a plurality of the fieldimages overlap with a transition period of the pixel shifting meansduring which the beam of said each pixel is shifted, and the displaycontrol means controls display of the field images so as to minimize anamount of overlap of the transition period with one of the displayperiods corresponding to a color having a higher spectral luminousefficiency than the color of any other of the display periodsoverlapping the transition period.
 3. A field sequential color displayapparatus for sequentially displaying field images corresponding to aplurality of colors to display a plurality of color frame images,wherein each of the field images is a single-color image and each ofsaid color frame images is formed from a plurality of the field images,said field sequential color display apparatus comprising: display meansfor sequentially displaying the single-color field images one-by-one;display control means for controlling a display order in which the fieldimages are displayed by the display means; pixel shifting means,including a liquid crystal cell, for shifting alternating ones of thecolor frame images, by shifting a beam from each pixel displayed on thedisplay means for a shifted one of the color frame images; and opticalmeans for transmitting to a viewer both shifted color frame images andunshifted color frame images; wherein the display control means controlsdisplay of the field images such that each said color frame imageincludes an undisplayed period, during which nothing is displayed forone field period, at a beginning or an end of the plurality of fieldimages making up the color frame image, so as to prevent color mixtureof the field images due to delay in response by the pixel shiftingmeans.